The present invention relates generally to ion implantation systems, and more particularly, to beam stops that serve to absorb the energy of an ion beam.
Beam stops are routinely utilized in ion implantation systems to intercept those portions of an ion beam directed towards a target, e.g., a semiconductor wafer, that do not impinge upon the target. The beam stop is typically incorporated into the end station of an ion implantation system, lying beneath or behind the wafers that are held in the beam path by the end station. The impact of ions on the beam stop typically causes a loss of material from the beam stop surface via a process commonly known as sputter erosion. This sputtered material can deposit on various surfaces of an evacuated chamber of the implantation system, including those of the wafers, thereby contaminating these surfaces. Performance of semiconductor devices formed on processed wafers can be adversely affected even by small amounts of contamination. For example, contamination of silicon wafers by metals or alkali metals is particularly pernicious.
Accordingly, beam stops are preferably formed from materials that exhibit low sputter erosion rates and/or eject sputter products that are less likely to contaminate the wafers. One such beam stop material is silicon, since the ejected silicon is less likely to adversely affect wafers of like composition. However, forming an entire beam stop from a material, such as silicon, that does not cause significant contamination is often impractical. The typical high current ion implanter can subject a beam stop to high power levels, for example, in a range of 5 kW to 15 kW. A beam stop that is exposed to such high power levels needs to be efficiently cooled but the brittle and refractory nature of silicon make it difficult to construct passageways for circulation of cooling fluid in the silicon itself.
Thus, the typical silicon beam stop needs to be coupled to a base, formed from a metal or similar material, which can be machined for cooling ducts and readily cooled. However, the exposure of the beam stop to high power levels can render bonding of the tiles to the base difficult. Further, the differences between the thermal expansion of the silicon relative to the base can exert substantial strain/stress forces on the silicon. These stress/strain forces can lead to mechanical failures, such as fractures, of the silicon tile that is exposed to the beam.
Accordingly, there is a need for beam stops that can be used in ion implantation systems without causing contamination.
There is also a need for beam stops that can withstand exposure to high power levels in ion implantation systems.
The present invention provides a beam stop for use in an ion implantation system that can employ multiple tiles to cover the beam-receiving surface. The beam stop further includes a base formed of a thermally conductive material, and a heat transfer layer formed of a semi-elastic material that is disposed on a surface of the base. The beam stop""s surface includes one or more tiles, each formed of a thermally conductive refractory material, that are disposed on the semi-elastic, heat transfer layer so as to face an ion beam in the implantation system. The heat transfer layer transfers heat generated in the tiles, in response to ion beam impact, to the base. A heat sink, e.g., a cooling fluid, coupled to the base removes heat from the base.
In another aspect, the thickness and the thermal conductivity of the base, and those of the heat transfer layer and the tiles are chosen so as to ensure uniform expansion of the base and the tiles when the beam stop is heated by ion beam impact This aspect of the invention is applicable to most xe2x80x9csingle tilexe2x80x9d and xe2x80x9cmulti-tilexe2x80x9d structures.
In a related aspect, the thermal expansion coefficients of the base and the tile, and their respective thermal conductivity, together with the width of the heat transfer layer, are selected such that a temperature change of the tile (xcex94Tt), in response to heat generated by ion impact, is related to a corresponding temperature change of the base (xcex94Tb) in accord with the relation:
(xcex94Tt)xcex1t=(xcex94Tb)xcex1b
wherein xcex1t and xcex1b represent thermal expansion coefficients corresponding to the tile and the base, respectively.
In one embodiment, the base can include at least one inner passageway that allows a cooling fluid, such as, water, to circulate through the base to remove heat that is transferred to the base via the heat transfer layer. The passageway can be formed, for example, as a channel at a selected depth below a surface of the base that is in contact with the heat transfer layer.
In further aspects, the invention provides a beam stop as described above in which the tiles and the base are formed of silicon and aluminum, respectively, and a silver-filled epoxy is utilized as the heat transfer layer. The aluminum base can include a channel for fluid circulation, formed therein at a depth in a range of about 0.002 to about 0.006 inches (0.015 cm) below a surface of the base that is in contact with the epoxy layer. Those having ordinary skill in the art will appreciate that other metallic materials, such as, titanium, nickel, copper, silver and various metal alloys, can be utilized for forming the base, and other conductive refractory materials, such as, graphite or germanium, can be employed for forming the tiles.
In other aspects, the invention provides a magnetically suppressed or an electrically suppressed Faraday cup that utilizes a beam stop according to the teachings of the invention. For example, such a Faraday cup can include a beam stop of the invention and a magnet that is coupled to the beam stop so as to maintain an electrically conductive path thereto. The magnet generates a magnetic field in a space above the tiles so as to capture secondary electrons that are emitted by the beam stop in response to the ion beam impact. The Faraday cup further includes an insulator coupled to the magnet for providing electrical insulation of the cup, and a circuitry coupled to the beam stop for measuring a current induced by ions absorbed by the beam stop.
In a related aspect, the invention provides an electrically suppressed Faraday cup in which a beam stop according to the teachings of the invention is electrically biased, for example, by a voltage source, so as to generate an electric field in a space above the tiles in a manner that the interaction of the secondary electrons emitted by the beam stop with the field causes the return of these electrons back to the beam stop. An electrical circuit can be provided to measure a current induced as a result of absorbance of ions incident on the beam stop.
Further understanding of the invention can be obtained by reference to the following detailed description in conjunction with the drawings which are briefly. described below.